1. Field of the Invention
The present invention relates to a flexible tube of a thermoplastic resin having poor melt flowability, such as ultra-high-molecular-weight polyethylene and flouroplastics. The present invention also relates to a method and apparatus for producing a tube from a thermoplastic resin having poor melt flowability.
2. Description of the Related Art
Flouroplastics and ultra-high-molecular-weight polyethylene are widely used as so-called engineering plastics because of their excellent impact resistance, abrasion resistance, self-lubrication, chemical resistance, and other properties. However, these plastics have extremely high melt viscosity and poor flowability when compared with conventional resins, so that it is very difficult to mold them by conventional extrusion molding and injection molding. Accordingly, fluoroplastics and ultra-high-molecular-weight polyethylene are generally molded by compression molding. However, continuous lengths such as pipes and rods cannot be substantially molded by compression molding. A ram extruder provided with a mandrel has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 55-28896, as an apparatus for continuously producing pipes of ultra-high-molecular-weight polyethylene. However, as is known in the art, ram extruders have disadvantages in that the molding rate and accuracy in the uniform section are poor. Thus, good pipes cannot be obtained. Furthermore, the molding of flexible tubes having a small outer diameter and a thin wall thickness is impossible. Although the molding rate can be increased by using a screw type extruder, neither good pipes nor flexible tubes can be molded from ultra-high-molecular weight polyethylene. Thus, since ultra-high-molecular weight polyethylene has extremely high melt viscosity and poor flowability as mentioned above, adjustment is needed in the uniform section and the resin is not sufficiently fused when a pipe forming die for conventional resins is used. Furthermore, sizing (i.e., regulation of the outer or inner diameter) cannot be effected by a conventional vacuum former or inner pressure controlling, internal stress remains after quenching, and therefore, the physical properties of the molded pipes are unpreferably decreased.
On the other hand, it has been proposed in, for example, Japanese Examined Patent Publication (Kokoku) Nos. 40-6993 and 43-16859 and Japanese Unexamined Patent Publication (Kokai) No. 51-137759, that a mandrel be rotated against a die in the production of thermoplastic resin tubes. However, when this method is applied to ultra-high-molecular-weight polyethylene, the desired flexible tube of ultra-high-molecular-weight polyethylene cannot be obtained since ultra-high-molecular-weight polyethylene has an extremely high melt viscosity and poor flowability, unlike conventional polyolefins. For example, when the die head having a flow path such that a molten resin partially flows in a direction opposite to the extrusion direction, as disclosed in Japanese Examined Patent Publication No. 40-6993, is used, extrusion molding is impossible. Furthermore, when an apparatus having a crosshead, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 43-16859, is used, the molten resin separated at the crosshead cannot be fused in the die at all. When an apparatus having a circular orifice provided between the screw tip and the die, as disclosed in Japanese Unexamined Patent Publication No. 51-137759, is used, the back pressure becomes extremely high and, therefore, extrusion moldability is poor and the desired product cannot be obtained. As mentioned above, the desired flexible tube cannot be obtained from a thermoplastic resin having poor melt flowability such as ultra-high-molecular-weight polyethylene and fluoroplastics by extrusion molding.